1. Field of the Invention
This invention relates to a process which by controlling the oxidation of nitric oxide permits reduced emission of nitrogen oxides and increased yield in the production of ammonium nitrite and alkali metal nitrites.
2. Description of the Prior Art
The absorption of nitrous gases into an absorption medium is an important step in a variety of chemical processes. Among these are processes for the production of nitrites, nitrates and nitric acid. Furthermore, nitrogen oxides (NO.sub.x) constitute undesirable by-products of many industrial processes. If emitted to the atmosphere, NO.sub.x is an environmental pollutant. In particular, NO.sub.x initiates a complex series of reactions which results in photochemical smog. Consequently, a great deal of research has been directed toward controlling NO.sub.x absorption into various absorption media.
U.S. Pat. No. 1,070,070, issued Aug. 12, 1913, to Rothe discloses a process for producing nitrite from nitrous gases containing air or oxygen by absorption in an alkaline medium. The patent discloses a method of increasing the quantitative yield of nitrites, which consists in subdividing the current of gases to be absorbed into separate gas currents on the way from the place of production to the absorbing medium, and so regulating the conditions of time and temperature in the respective subdivided gas currents that the nitrous constituents in one portion are delivered into contact with the absorbing medium substantially as NO.sub.2 and nitrous contituents in the other portion are delivered into the absorbing medium substantially as NO, the gas currents being again commingled in such manner that at the moment of absorption a sufficient quantity of NO is present to satisfy the molecular ratio of NO+NO.sub.2 =N.sub.2 O.sub.3.
U.S. Pat. No. 1,061,630, issued May 13, 1913, to Pauling and U.S. Pat. No. 3,453,071, issued July 1, 1969, to Schmitt, et al., disclose processes for producing substantially equimolecular mixtures of NO and NO.sub.2 for absorption by means of suitable absorbing agents.
None of these patents teaches a process that overcomes the serious problem of environmental pollution by No.sub.x emission. Thus there is a need for a process that provides controlled oxidation of nitric oxide to yield maximum gas absorption in a basic solution and minimum NO.sub.x effluent.
An important commercial process that yields NO.sub.x as a byproduct is ammonium nitrite production. Large tonnages of ammonium nitrite are produced for use in the production of hydroxylamine compounds, which in turn are used in the manufacture of caprolactam via an intermediate, cyclohexanone oxime, and in the manufacture of specialty oximes. Ammonium nitrite is produced by contacting in an absorption tower an aqueous system containing a basically reacting ammonium compound with a gaseous stream containing NO, NO.sub.2, oxygen and inert gas.
U.S. Pat. No. 4,045,540, issued Aug. 30, 1977, to R. R. Hertzog, discloses a process for reducing NO.sub.x emission in ammonium nitrite production by adding NO.sub.2 to the gas being absorbed in the absorption tower to reduce the mole ratio of NO:NO.sub.2 from at least 2.0 (in the entering gas) to between 0.8 and 1.0 (in the exiting gas). The NO.sub.2 is generated by oxidation of part of the NO present in the entering gas. If too much oxidation takes place, production of ammonium nitrate reduces the yield of ammonium nitrite.
Although Hertzog's process permits substantial reduction in NO.sub.x emission, it does not provide adjustments in oxidation to compensate for changes in operating conditions, such as throughput and composition of the feed gas and temperature and pressure in the absorption tower. Specifically, changes in interfacial gas-liquid contact area and/or gas residence time and partial pressures considerably affect the value of the ratio and make it virtually impossible to insure absorption of the nitrogen-oxygen compounds to below 1000 ppm NO.sub.x in the effluent vent gas. Changes affecting the production capacity of the unit result in changes in oxygen consumption which relate directly to the oxidation state of the fixed nitrogen in the aqueous product and exit gas streams.